scan showed problem with cylinder 4 on my truck. Mechanic said it shouldnt be running that bad even if was only running on 7 cylinders. He took 2 hrs to dismantle exhaust lines to check for a problem there, installed an O2 sensor, finally pulled the spark plug to check compression on that cylinder which was basically none. Clearly this should have only cost me 15 min labor to pull plug. SCAN SAID CHECK CYLINDER 4. 2 hours labour and 68 for sensor to find engine dead. STAY FAR AWAY!!!!!!!!!!!!!!!